Investigation and Application of “Bluff-body in Cavity” Burner for Pulverized Coal Combustion

Dev. Chem Eng. Mineral Process., 9(3/4), pp.357-370,2001.

Investigation and Application of “Bluff-body

in Cavity” Burner for Pulverized Coal

Combustion

Gang Chen*, Ji-Hua Qiu, Ming-Hou Xu,

and Chu-Guang Zheng National Laboratory of Coal Combustion, Huazhong University of Science and Technology, WubnY Hubei, 430074, P, R. China

The flow and combustion process of a new type of pulverized coal burner, the

“bluff-body in cavity”, is studied in this pap@. This is an improvement on the basic

principle of the ordinary bluff-body bur=. Mean and fluctuating velocity

components and turbulence characteristics of the flow in the outlet of the “blfl-body

in cavity” burner were measured using a three-dimensional laser particle dynamics

anemometer (3D-PDA). Combustion tests showed that this burner is better than an

ordina?y burner with only a bluff-body regarding the ignition and flame stability.

Application of this new burner in several power plant boilers (65-670 t/h) showed that

the temperature in the flame zone is high, the combustion process is very stable, and

the boiler efficiency is increased. These improvements indicate a promising fiture

for the burner.

Keywords: Coal combustion; burner; bluf-body; flame stabilization.

* Author for correspondence (gangchen@mail. hust.edu.cn).

357

Gang Chen, Ji-Hua Qiu, Ming-Hou Xu, and Chu-Guang Zheng

Introduction

The effective utilization of coal has become an important issue in energy development

worldwide. At present, direct combustion of pulverized coal, mainly in utility and

industrial boilers, is the most prevalent method of coal utilization. In China, the coal

reserve is very large, and annual production is over 1.2X lo9 tons. However, the

type and grade of coal vary widely, and it is necessary to utilize all the different coals

to meet the energy needs of the country. Combustion technologies for high-grade

bituminous coal have been well developed, but there are still numerous combustion

problems associated with the burning of coals with low volatile matter and high ash

content, as well as the burning of lignite and anthracite. These problems include

difficult ignition, poor flame stability and low combustion efficiency. Thus the

efficient use of low-grade coal in power plants needs further study. By considering

the characteristics and functions of the many burners so far designed for boilers, it can

be established that the design requirements for a pulverized coal burner for low-grade

coals are as follows [ 1-41 :

(1)

(2) stabilizing and strengthening combustion;

(3)

(4) low emissions of pollutants.

As a means of stabilizing low-grade coal combustion, the bluff-body burner,

which meets most of the above requirements, has been widely used in many utility

boilers throughout China [5 ] . The coals burned include low-grade bituminous coal,

sub-bituminous coal and anthracite. It has been shown that the bluff-body burner

no (or less) oil for ignition;

stabilizing combustion under low boiler load;

358

"Bluff-body in Cavity" Burner for PCC

can not only reduce coal and oil consumption but also stabilize the combustion

process during low-capacity operation. However, for those low-grade coals that are

more difficult to ignite, the recirculation zone and the recirculation rate created by the

bluff-body are not large enough, and the bluff-body is easily abraded or even burnt

out or disintegrated since it is located in the high-temperature flame region in the

furnace. Therefore, it is necessary to develop a novel burner with larger

recirculation zone and higher recirculation rate, and in which the bluff-body is

sheltered to ensure that the utility boilers can be operated safely and economically.

To satisfy the above conditions, a new kind of pulverized coal burner has been

designed, namely the bluff-body in cavity burner.

The bluff-body stabilizer [6, 71 is capable of engaging the high-temperature flue

gas from the centre of the furnace to the inlet area of the primary air through the

recirculation flow behind the bluff-body, as shown in Figure 1. Because of the high

temperature of the recirculation flow, the ignition and combustion are stren,gthened

and stabilized. The bluff-body also produces considerable disturbance to the coal

and air flow, increasing heat and mass transfer between the low-temperature coal-air

flow and the high-temperature flue gas. Many experiments and industrial tests have

proved the effectiveness of the bluff-body stabilizer, which depends to a great extent

on the size of the recirculation zone and the recirculating flow rate. However, the

bluff-body stabilizer also has drawbacks in that its recirculation zone is not long

enough for coals of very low grade, as mentioned above. Hence it is necessary to

adopt measures to strenagthen the recirculation flow behind the bluff-body. The

bluff-body in cavity pulverized coal burner is simply a burner with a cavity around

359


the bluff-body, and it is shown schematically in Figure 1 (wl indicates the outlet

velocity of the primary air; B and b indicate the width of the primary air nozzle and

the bluff-body, respectively).

When the coal-air mixture flows past the bluff-body, a low-pressure region is

formed behind the bluff-body, and the high-temperature flue gas is recirculated

towards this region, thus enhancing the ignition of the coal. Since the height of the

bluff-body is limited, a pressure gradient caused by the low-pressure region occurs

not only at the curvature behind the bluff-body but also over the height of the

bluff-body, as shown in Figure 2. However, the latter entrains the lower-temperature

gas, unlike the former which entrains flue gas of higher temperature. The cavity

bluff-body burner was developed to avoid the entrainment of high temperature gas

over the height of the bluff-body, so as to enhance the recirculation of the

high-temperature flue gas.

Experimental Details

The cold flow tests were conducted in a closed wind tunnel with a testing section

0.26 m wide, 0.18 m high and 1.5 m long. The width of the triangular bluff-body is

5cm. The free stream has a cross-section of 15 by 10.5 cm. In the wind tunnel the

free stream velocity can be varied from 10 to 50 ms". The experimental set-up is

illustrated in Figure 3.

The pilot-scale pulverized coal furnace used for the combustion tests is shown in

Figure 4. The furnace is 4m long and has a cross-section of 0.35m by 0.50 m. The

primary and secondary air flow streams through a preheater. The temperature profile

360


n

Figure 1. Schematic diagram of (a) bluf-body burner; (3) cavity bluff-body burner

Figure 2. Entrainment of gas at top and bottom of bluff body

(1. recirculating flue gas; 2. compensating gas).

361


No. 1 No. 2

Table 1. Proximate analyses of tested coals.

Mad Aad v a d cad QLT..CLC"

0.94 24.57 12.36 63.13 24620 1.19 44.79 8.28 45.74 16610

Note: Md Aam Vad and Cad are coal moisture, ash, volatile content, and fued carbon (%; as received), respectively; Qacnerm (k/kkgl is coal net calorific value (as received).

along the centre line of the fiunace was measured accurately with thermocouples,

fiom which the combustion characteristics of the burner could be analyzed. The

proximate analyses of two coals tested in this study are given in Table 1.

Particle-dynamics anemometer (PDA) was used in this study based on Phase

Doppler Anemometry, which is an extension of Laser Doppler Anemometry (LDA).

The instrument includes an argon ion laser, transmitter, fibre optics, receiving optics,

signal processor, traversing system and computer system. PDA uses the proven

phase doppler principle for simultaneous non-intrusive and real-time measurements of

three velocity components and turbulence characteristics. It makes use of a new

method of determining the phase differences between doppler signals received by the

three detectors located at different positions. The transmitting optical signal is based

on 55X Modular LDA optics. Several optical confi,grations are available with

measuring distances from 50 to 600 mm. All instrument settings, such as bandwidth

and high voltage, are controlled by computer. An analogue-digital converter allows

the computer to read the anode current of the photomultipliers. The combination of

photomutiplier and particle velocity correlation bias can contribute to uncertainty, but

the error is likely to be small. The overall uncertainty in measured values of mean

velocity and particle diameter are 1% and 4% respectively, and the range of

measurable sizes is 1 pm to 10 mm.

362


1

6

Figure 3. Schematic diagram of cold flow test apparatus: 1. fan; 2. transition section; 3. particle injection point; 4. test section; 5. wind balance chamber; 6. recirculation section.

3

1

Figure 4. Schematic diagram of combustion test apparatus: I . blower; 2. air preheater; 3. air pipe; 4. secondary air distributor; 5. pulverized coal hopper; 6. pulverized coal feeder; 7. primary air valve; 8. electric motor; 9. secondav air valve; 10. thermocouple; 11. ficrnace; 12. sampling points; 13. cyclone separator

363


Results and Discussion

I ColdfIow tests

The frst problem was the choice of particles to model pulverized coal. Talc and

alumina were rejected for this purpose and white polychloroethylene powder was

selected. This has a density of 1.2-1.4 gkm3 and a particle size of < 160 pm, both

similar to those of pulverized coal.

II Recirculation characteristics

Fiewe 5 shows the length and the width of the recirculation zone of both the cavity

bluff-body burner and the ordinary bluff-body burner. The relative len,@ of the

recirculation zone, i.e. W2b (2b is width of the bluff-body, L is length of the

recirculation zone) of the former is twice that of the latter, while the relative width,

W2b (R is half the width of the recirculation zone) of the recirculation zone of the

former is approximately 1.6 times that of the latter. Figure 6 shows that the

recirculation rate, m/mo (mo is mass flux of the primary air, m is .mass flux of the

reverse flow), of the former is twice that of the latter. The increases in these

parameters are very beneficial in intensifying the coal combustion process [8,9].

III Turbulence intensity

The intensity of turbulence is very important in pulverized coal combustion. Usually,

the turbulence intensity (u’ is fluctuating velocity, w1 is velocity of the = GI,,,, primary air) of a ffee jet is not large, at most approximately 15%. With the

bluff-body, the turbulence intensity has double peaks, one near the edge of the

364


I100

'. 8 - I ; LOO

7 0 0

1.5

c 1.0 m 0.8

D

-

I 2 3 4 L12b

Figure 5. Length & width of recirculation zone with cavity bla-body burner ( )

and ordinary blq-body burner ( A ).

301

I I I 0.4 0.8 I .2

Y12b

Figure 7. Turbulence intensity ( 0 separatedflow; Xpee jet).

V.7, I

Figure 6. Recirculation rate with cavity big-body burner ( ) and ordinary bluf-body burner ( A ).

Figure 8. Temperature along furnace center (cm). Coal No. I ; Coal N0.2

365

Gang Chen, Ji-Hua Qiu, Ming-Hou Xu, and Chu-Gmg Zheng

recirculation zone, and another smaller one far fiom the axis in the radial direction,

with almost the same value as that of the fiee jet (see Figure 7). This is because the

turbulence intensity is due to both the gas flow and the solid wall.

IV Combustion tests

The experimental results are shown in Figure 8. The temperature distribution along

the furnace centre is similar. Note that coal number 2 is a low rank anthracite with

very poor ignition and burnout properties. The test results show that the combustion

is very stable without oil co-combustion for the novel burner, while oil co-combustion

is necessary for the ordinary burner. This shows that the bluff-body in cavity is very

useful for consolidating ignition and stable combustion.

The main reasons for improved characteristics of the bluff-body in cavity over the

ordinary burner can be evaluated. As discovered by the cold flow tests, both the

relative length and width of the recirculation zone of the novel burner are larger than

for the ordinary burner. Meanwhile, the recirculation rate of the bluff-body in cavity

burner is also higher. Note that the temperature is much higher in the recirculation

zone, which means easier ignition and better combustion stability for the fiesh

primary air flow with a much lower temperature. Also consider the effects of the

turbulence intensity on the performance of different burners. If a higher turbulence

intensity exists, such as for the bluff-body in cavity burner, the mass and heat

exchange among gas species and coal particles is enhanced. In these circumstances,

small coal particles rapidly obtain energy from high-temperature flue gas in the

recirculation zone, and large coal particles will be easily ignited immediately after the

366

"Blufl-body in Cavity" Burner for PCC

smali particles are ignited and combustion occm. In the opposite case, for the

ordinary burner with poor turbulence characteristics, the ignition and then fi,uther

combustion will be difficult because of the lack of good mass and heat transfer.

V Application of bluff-body in cmity in industrial utiIW boilers

Table 2 lists some results for particular applications, such as boiler efficiency and the

lowest load without oil co-combustion, for different coals in several power plants in

China. The boiler efficiency and the lowest load without oil, with and without the

bluff-body with cavity, are tabulated and compared. These results show that the

bluff-body with cavity burner has better behavior during coal ignition and the

combustion process, and for flame stabilization.

Conclusions

The len,@ and diameter of the recirculation zone and the recirculation rate in the

outlet of the bluff-body in cavity burner are higher than those of the ordinary

bluff-body burner.

Close to the boundary of the recirculation zone, there is an intensive velocity

fluctuation, which creates the heat-mass exchange. Therefore, it is very helpful

in heating the primary flow, and makes the ignition easier and combustion stable.

The technique of flame stabilization through the bluff-body in cavity burner can

be used with different types of coals and different rated capacity boilers.

Industrial applications have shown excellent capabilities for this burner in flame

stabilization and combustion intensification, especially with low rank coals.

367


I

Table 2. Boiler efficiency and lowest self-stabilizing load

Proximate analysis of current coal

As above.

Boiler efficiency (%)

Ordinary Novel burner 1 burner

81 88

1 90.2 85

87 88

83 89

90 1 91.8

Difference

7

5.2

I

6

2

1.8

Units: v a d 34.8% ; Md 5.20% ; Ad 24.6% ; Qu.net,cv kJkg

368


Lowest self-stabilizing load (without oil co-combustion) (%)

Table 2 contd.

Boiler performance

Difference

30

25

Ordinary burner

90

(S.H. = superheater)

Rated capacity 220 t/h, rated output 50 MW.

Ordinary burner: S.H. outlet temp. 460%;

Novel burner: S.H. outlet temp. 540%

(as designed)

Rated capacity 670 t/h, rated output 200 MW

Ordinary burner: furnace wall serious slagging

Novel burner: slagsing problem solved

Rated capacity 75 tfh, rated output 12 MW

Ordinary burner: the fuel is bituminous, coal

proximate analysis as follows:

M,d=3.5%, Vd=28.6%, Qar,ncsp=20980kTkg

80

45

l5

70

80

Ordinary burner: furnace wall serious slagging

Novel burner: slagging problem solved

Rated capacity 420 t/h, rated output 125 MW

Ordinary burner: WR burner is used

Rated capacity 670 t/h, rated output 200 MW Ordinary burner: furnace upper wall serious

slagging

Novel burner: slagging problem solved

85

60

Novel burner

60

55

65

60

40

45

2o 1 Rated capacity 130 tm, rated output 25 MW I

369

Gang Chen, A-Hua Qiu, Ming-Hou Xu, and Chu-Guang Zheng

Acknowledgments

The financial support of the Special Funds for Major State Basic Research Projects

(G19990222 12-05> is gratefully acknowledged.

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Received: 30 April 2000; Accepted @er revision: 10 April 2001.

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Investigation and Application of “Bluff-body in Cavity” Burner for Pulverized Coal Combustion